olivier Posted January 21, 2011 Report Share Posted January 21, 2011 Hi, what is the best way with the PID FB (auto tuned) to implement a regulation with "heat and cold" i mean with two physical output one to made the measure grow the other that helps the measure to decrease ? For now i set the CV low to -1000 and the CV high to +1000 and done some code to activate each output via two PWM FB ... Is it the right way? is thre another better method? Thank you Olivier Quote Link to comment Share on other sites More sharing options...
Emil Posted January 21, 2011 Report Share Posted January 21, 2011 What is your system? Why you want to perform heating and cooling? Quote Link to comment Share on other sites More sharing options...
olivier Posted January 21, 2011 Author Report Share Posted January 21, 2011 The process is to regulate a pH by injecting air with an output and CO2 with the other... i am using a pwm fb for each output. Those output are electrovalve. Thank you Quote Link to comment Share on other sites More sharing options...
Deke Posted January 22, 2011 Report Share Posted January 22, 2011 Hi olivier & Emil, Attached is subroutine, in this subroutine it controls one zone of a barrel that has a screw in it (Plastic Molding Machine), it does both heating and cooling and works very well. It does not use the PID. Deke I was unable to upload a file with the extension vlx, which is the exported subroutine file, so I had to make a new app with just that subroutine in it. 1 Quote Link to comment Share on other sites More sharing options...
dahnuguy Posted January 23, 2011 Report Share Posted January 23, 2011 What is your system? Why you want to perform heating and cooling? To control a temperature I would imagine. Like with a 2 way heat exchanger. Steam and condensate piped in one way, cold water and drain the other way. Holds temperature to a setpoint like heating 2000 gallons of liquid to 200 deg F and then hold it there without going over for a period of time, them bring the temperature down in steps or b a ramp to 100 degreees F and hold it there, then cool it down to 70 degrees F and hold it there. You can't do that with a one way PID. I would like to control a ,motor speed with PID both acceleration and deceleration. Currently as I understand the Unitronics PID, we can control up to a setpoint and once the PV exceeds the setpoint, the output of the PID just stops. Which would work for heat only. But does not control the motor very well. Nor would it stop the temperature from continuing to climb above setpoint. What would you do in Visilogics to heat to a setpoint and then stop the temperature from continuing to climb? Just wait and hope it cools off by itself? What about a chemical process that needs heating and cooling to maintain control? Is this one way PID control just a limit of the software? Or do I not understand it correctly? Quote Link to comment Share on other sites More sharing options...
Ziwi Posted January 23, 2011 Report Share Posted January 23, 2011 Hi Olivier I have used 2 PID routine's, one heating, one cooling and logic to select which one runs. Eg - If in heating mode and PV is above set point for a certain time, switch to cooling mode. My application is a cooling tower for a moulding shop. I have a speed controlled fan for cooling, and a bypass valve for heating. Although this system has been working ok for over a year, the changeover - heating/cooling is not a smooth one. My new approach is to PID control the fan, then the valve attempts to keep the fan speed within a certain range. So if fan speed goes outside of a wide dead band, make adjustment to valve position. I also add a forward feedback proportional value onto the PID CV to give a final CV. This improves control as the cooling tower receives large temp spikes. Quote Link to comment Share on other sites More sharing options...
antoinezerafa Posted April 21, 2017 Report Share Posted April 21, 2017 On 22/01/2011 at 10:26 PM, Deke said: Hi olivier & Emil, Attached is subroutine, in this subroutine it controls one zone of a barrel that has a screw in it (Plastic Molding Machine), it does both heating and cooling and works very well. It does not use the PID. Deke I was unable to upload a file with the extension vlx, which is the exported subroutine file, so I had to make a new app with just that subroutine in it. Hi Deke Can you connect a thermocouple directly to the plc? Can you send me the routine pls Quote Link to comment Share on other sites More sharing options...
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